Influenza is a leading cause of death among American adults. Each year, about 36,000 people die from influenza, and more than 200,000 people are hospitalized. Influenza is a highly contagious disease that is spread by coughing, sneezing and through direct physical contact with objects that carry the virus such as doorknobs and telephones. Symptoms of influenza include fever, extreme fatigue, headache, chills and body aches; about 50 percent of infected people have no symptoms but are still contagious. Immunization is 70-90 percent effective in preventing influenza in healthy people under the age of 65, as long as the antigenicities of the circulating virus strain match those of the vaccine.
Vaccination is the main method for preventing influenza, and both live attenuated and inactivated (killed) virus vaccines are currently available. Live virus vaccines, typically administered intranasally, activate all phases of the immune system and can stimulate an immune response to multiple viral antigens. Thus, the use of live viruses overcomes the problem of destruction of viral antigens that may occur during preparation of inactivated viral vaccines. In addition, the immunity produced by live virus vaccines is generally more durable, more effective, and more cross-reactive than that induced by inactivated vaccines, and live virus vaccines are less costly to produce than inactivated virus vaccines. However, the mutations in attenuated virus are often ill-defined, and reversion is a concern.